How Interesting. I spot-checked a few APs and see that I'm running 25w per AP. The ap reports no problem and has plenty of power. the AP and switch negotiated Power via LLDP.
Have you checked the cable runs by chance? the further they're run, the more power is lost to the AP.
To answer your question, yes, I have purchased POE injectors where I'm unable to feed them properly with the switches on site. Typically I use StarTech 4 port injector and do know that the unit cannot provide AT on all 4 ports reliably( but should work well for your need to power 3 APs).https://www.amazon.com/StarTech-com-Port-Gigabit-Midspan-Wall-mountable/dp/B00VRESOW4
161 days, 17:40:03
Cisco IOS Software, C3750E Software (C3750E-UNIVERSALK9-M), Version 15.2(3)E, RELEASE SOFTWARE (fc4)
Bridge, on;Router, off